In the manufacture and assembly of articles incorporating plastic lenses, for example lenses made of acrylic, polycarbonate, etc., such as in the manufacture and assembly of motor vehicle instrument panels and the like, the lenses often are assembled with other components such that at least one surface of the lens is not readily accessible for cleaning. Attempts generally are made, therefore, to remove lint, dust and like contamination from the surface of the lens before its incorporation into the assembly. Such decontamination of the lens surface can be done by any of several known means including, for example, wiping with a cloth. The surface may become again contaminated, however, as the result of any static charge on the surface of the part. Such static charge will develop over time under normal conditions. The static charge will attract contaminants, particularly airborne contaminants, to the lens surface.
Various processes and equipment are known and used to neutralize static charge at specific points on the surface of a work piece, such as ionizers, static bars, grounding brushes, etc. However, non-conducting materials, such as plastic, which typically accumulate large static charges quite readily, often cannot be effectively point discharged. Also, as the part moves through a production or assembly process, it may quickly pick up a static charge again, often only a short time after it has been neutralized. It often is impractical to place numerous grounding or ionizing devices throughout a production process. Also, antistatic protection for a material may be important to the product after it is assembled. None of the above-mentioned devices or methods provide significant antistatic protection to a part during end use of the assembled product.
Antistatic agents have been used for many years in various industries to reduce static problems during processing and use of plastic parts. They reduce the attraction of dust and dirt to the surface of the part by reducing the static charge on the surface of the part. Antistatic agents may be applied to the surface of the finished article or incorporated into the bulk polymer during processing. They are believed to function generally by decreasing the rate of static charge generation and/or by increasing the rate of charge dissipation. For plastic lenses, the incorporation of an antistatic agent into the bulk polymer of the part has been found in many cases, however, to decrease the transparency or otherwise to adversely affect the optical quality of the lens and for this reason is often an unsuitable approach. The application of an antistatic agent to the surface of a plastic article by dipping, wiping or spraying has been used effectively. For cost effective high volume manufacturing and assembling processes, however, dipping or wiping an antistatic agent onto the plastic part presents several difficulties. These processes tend to be time and cost inefficient and an open bath of antistatic agent can become contaminated over time.
An antistatic agent can be sprayed from a reservoir in a closed container, which will reduce contamination, but significant processing problems are presented. Typically, local plant air is used in the spray system and airborne contaminants may be entrained with the antistatic material and deposited onto the surface of the lens. Also, difficulties can be encountered in controlling the amount of material deposited on the surface of the part and the distribution of the material over the surface of the part. Typically, spray systems have been found unable to produce sufficiently fine atomization of liquid antistatic agent required for a uniform thin film coverage, which is important for preserving the optical qualities of a plastic lens. When an excessively heavy coating is applied, the coating will be slow to dry, may smear upon contact with another surface and may otherwise impair the optical quality of the lens. In addition, it has long been a problem associated with the spraying of antistatic material that the material, even if a suitable amount is applied, does not distribute itself evenly over the surface of the part, being heavily concentrated in certain areas and sparse in others. Insufficient coverage can even result in bare spots, i.e., areas on the lens surface not coated with the antistatic material. Sparsely coated and uncoated areas are found to have unacceptably high static charges. This results in uneven antistatic protection, poor appearance and poor lens optics.
One approach which has been tried for removing the static charge from a plastic lens is the use of a flow of ionized air. Commercial units are available which generate a flow of ionized air which can be used to blow contamination from the surface of a plastic lens and simultaneously deionize the surface. While this method has proven effective, its effects are not long lasting. That is, plastic lenses so treated have been found to have unacceptably high static charge levels within as little as several hours from the time of treatment. Hence, dust, lint and other contamination is attracted to the surface of the lenses during its assembly with other components and during shipment and use of the final product.
Accordingly, there is a need for a method and apparatus for providing a durable antistatic protection to the lens. Preferably, the method and apparatus would be suitable for high volume commercial production applications. These and other objects met by the invention, or in certain cases met by preferred embodiments of the invention, will be better understood in the light of the following disclosure and discussion of the invention.